After fabrication and dicing of devices formed on a wafer, the individual dice are picked up from the diced wafer by a collet in a pickup head and placed in a bonding position on a carrier, substrate or other device, for further processing.
To avoid the omission of dice from production units it is desirable to raise an alarm if the pickup head fails to pick up and place a die at the bonding location.
Known detection systems generally use a light source and a light sensor which are aligned and placed on opposite sides of the site where the presence or absence of an object, for example the die, is to be detected. The presence of the object at the site interferes with transmittance of a narrow beam of light (whether collimated or focused) from the source to the sensor. The detection by the sensor of light radiated from the source is used an indication that the object is not present at the site. Conversely, the absence of light radiated from the source and detected by the sensor is used an indication that the object is present at the site.
However, these known systems fail in the case of transparent, or even partially transparent, objects where light can be transmitted through the object, and a relatively strong signal can still be received by the sensor even when the object is present.